1. Field of the Invention
This invention pertains generally to oscillators, more particularly to oscillators found on frequency selective surfaces (FSS) or metamaterials, and most particularly to variable geometry oscillator elements such as inductors and capacitors found in oscillators comprising FSSs or metamaterials, typically using split-ring-resonator (SRR) elements.
2. Description of Related Art
Materials designed to exhibit specific certain frequency responses have been studied for many years. Before the term ‘metamaterials’ was coined, these materials were often called ‘frequency selective surfaces’ (FSS). The main differences between the two classes are: 1) metamaterials are often designed with simultaneous electric and magnetic responses, providing the ability to design both the material permittivity and permeability simultaneously.
FSS, in contrast are generally designed to be either inductive or capacitive in nature, thus creating a resonant behavior but permitting the diversity of design tuning options available in metamaterials. FSS necessarily comprise periodic arrays of sub-wavelength resonators (typically around ½ wavelength in extent). Ultimately this means that the fundamental approach to tailoring the material properties is different. In the case of FSSs, it is a ‘structural’ approach wherein periodically positioning scattering elements leads to wave interference phenomena that changes the effective behavior of the material as a whole. In the case of metamaterials a ‘materials’ approach is used wherein a material's constitutive parameters (e.g. permittivity, permeability) themselves are altered. Therefore, in metamaterials, there is no need to invoke wave interference phenomena on the material as a whole and the metamaterial's sub-wavelength resonators need not necessarily be periodically arranged.
There are broad similarities between FSS and metamaterials. FSSs, like metamaterials, are comprised of small resonator structures, also referred to as elements. Each resonator element responds to a specific frequency, which is part of why an array of identical elements behaves as a frequency selective surface, just like metamaterials. However, in FSS, the periodic arrangement of these identical resonators plays a crucial role in generating interference patterns with the electromagnetic wave of resonance. With metamaterials, a periodic arrangement is not required.
Significant levels of research are ongoing in the field of metamaterials, which custom designs allow for the tunability of material parameters such as permeability and permittivity. Recently, dynamical control of these parameters has been demonstrated, but to a very a limited extent. This control was demonstrated by using the so-called split-ring-resonators (SRR), where the resonant behavior of the metamaterial could be rapidly switched on or off by photodoping a semiconductor layer upon which the metallic SRR was fabricated.
Similarly, the resonant response can now be switched both on and off, and may even be amplitude modulated, by the manipulation of a depletion layer in the semiconductor underneath the SRR. These demonstrated dynamic tuning methods, however, will only allow one to switch on, switch off, or modulate the amplitude of the resonant response. Neither of these methods allow for the frequency tuning of the SRR response, which in these examples is entirely determined by the initial fabrication dimensions and layout of the SRR array. Currently, it appears that there is no method by which dynamic frequency modulation can be achieved in metamaterials or FSSs after their initial fabrication. Restating this, it appears that there is no way to alter the initially fabricated resonant frequency of SRR that comprise FSSs or metamaterials.
One potential way to tune the frequency at which a metamaterial exhibits an effective bulk material response would be to change the dimensions and/or geometric layout of the elements that comprise the metamaterial as a whole. While these elements are typically SRRs arranged in a periodic array, the idea is general to any resonant metamaterial structure in any arrangement. The tuning problem here is that these structures are generally comprised of metallic patterns fabricated on an insulator or semiconductor substrate. As such, there is no way to change their size or shape once they are fabricated.
Optical manipulation of FSS has been demonstrated in the literature as early as 1996 by optically stimulating a Si substrate upon which FSS resonant structures were fabricated. This permitted optical switching on and off of the resonant behavior of FSSs. (Vardaxaglou, Electronics Letters 32, pg. 2345, 1996). Again, this method of optical switching did not allow for resonant tuning.
Frequency tuning was similarly performed using FSSs grown on top of ferrite substrates. In this method of tuning, however, the substrate (upon which the FSS structures are grown) is altered via an externally generated magnetic field. When this substrate is altered via the externally generated magnetic field, the substrate then alters the resonant frequency at which the FSS material as a whole responds. (Chan, et al. Microwave and Optical Technology Letters, 13, pg 59, 1996). In the 1996 Chan, et al. paper, the entire substrate underneath the individual resonator structures was altered. By using the Chan substrate alteration method, only the entire substrate may be altered; selective areas of the substrate may not be altered to permit regional or individual resonant structure tunability. Additionally, Chan et al. must provide a highly uniform magnetic field in both strength and direction, otherwise the resonant behavior of the devices may be compromised.
Most other metamaterial or FSS tuning methodologies have involved fabricating voltage controlled lumped elements (such as varactors) disposed between the resonant structures. Varactors are frequently known as semiconductor devices where their capacitance is sensitive to the applied voltage at the boundary of the semiconductor material and an insulator. While voltage controlled lumped elements permit a change in the resonant frequency of the resonant structures, this method suffers from the high complexity of devices and is not as fast as the methods described here. See (Gil et. al, IEEE Trans. Circuits with these Microwave Theory and Tech. 54, pg 2665, 2006), (Mias, Electronics Letters 39, pg 724, 2003), (Reynet et. al, Applied Physics Letters 84, pg 1198, 2004). The Gil methods were only shown for coin-sized microstrip circuits in only one dimension, which is hardly relevant to a close-packed FSS or metamaterial requirement for high frequencies in at least two dimensions. Additionally, the complexity introduced by the voltage controlled lumped elements requires additional photolithographic steps, and further requires more substrate real estate, thereby limiting device density.
Other methods of FSS tuning or metamaterial tuning involve mechanically moved elements, a tremendously slow method compared to charge carrier lifetimes, which can be at a level of sub-picoseconds (ps). See (Tsakonas, Microwave and Optical Technology Letters 48, pg 53, 2006), and (Zendejas et al., Journal of Microelectromechanical Systems 15, pg 613, 2006).
Another method of FSS tuning involves varying the thickness of the superstrate deposited over a substrate (Chandran, A. R., et al., Electronics Letters, 2004, Vol. 40, No. 20). This method is only able to tune to a single frequency during initial fabrication, and testing of the tuning is only possible after completed fabrication.
U.S. Pat. No. 6,911,957 (the '957 patent) discloses a method for dynamically varying a frequency response of a frequency selective surface (FSS). The method can include controlling transmission of electromagnetic energy through a frequency selective surface by passing selected frequencies in a pass-band and blocking selected frequencies in a stop-band. The stop-band and the pass-band can be dynamically modified by controlling at least one of a position and a volume of a conductive fluid that forms a portion of the frequency selective surface. According to one aspect of the method, the conductive fluid can be selected to include gallium and indium alloyed with a material selected from the group consisting of tin, copper, zinc and bismuth.
In the '957 patent, the resonance frequency of a FSS is tuned by injecting a conductive fluid into an array of resonators. FSS are very similar to the resonator structures that form modern metamaterial structures, but important differences do exist, particularly in regard to the magnetic resonance achieved by metamaterial resonators. The introduction of a slow-moving conductive fluid for tuning purposes renders this method exceedingly slow for nearly all frequency tuning applications. What is needed is a method for quickly switching frequencies on electronic timescales (ns, fs, ps, sub-ps, etc.), not on the order of seconds or minutes (at least nine orders of magnitude slower).
U.S. Pat. No. 6,232,931 (the '931 patent) describes an optically controlled frequency selective surface (FSS) that includes an electrically conductive layer having an array of radio frequency scattering elements such as slots formed in an electrically conductive layer or pattern of loops mounted to a substrate. Photonically controlled elements, such as photo-diodes, photo-transistors, and other photo-electronic devices, are connected across each of the scattering elements. Electromagnetic characteristics of the FSS, including resonant frequency, impedance, and the pass/stop band, may in turn be modulated by controlling the degree of illumination of the photonically controlled elements.
The '931 patent does describe a method of tuning the resonant frequency of a FSS surface by means of optical illumination. However, it seems to be limited to only radio waves, and describes a method of altering resonant structures by means of the insertion of photosensitive devices (or lumped elements such as transistors) within individual slotted resonators, and not by modifying the geometry of any component inductor or capacitor. The '931 patent apparently only discusses FSS and does not appear to discuss metamaterials. It does appear that all of the photo-controlled elements are selected from the group that includes bulk semiconductor switched, photocells, photodiodes, phototransistors, and photovoltaic controlled field effect transistors.
What is needed is a method for dynamically frequency tuning small resonant elements, either on FSS or metamaterials or discretely, that does not require complicated device fabrications within slotted resonant structures, and therefore could be applied to much smaller resonant elements and thereby operate at higher frequencies. Ideally, one would be able to change material fundamental constitutive parameters, not just change in the resonance of wave interference phenomena. Such a device could be very important as metamaterials progress to higher and higher frequencies (e.g. visible light) where strict periodic placement of sub-wavelength resonators required in FSS is likely to become difficult, if not impossible.